Pattern measuring method and pattern measuring device

ABSTRACT

A pattern measuring method and device are provided which set a reference position for a measuring point to be measured by a scanning electron microscope and the like, based on position information of a reference pattern on an image acquired from the scanning electron microscope and based on a positional relation, detected by using design data, between the measuring point and the reference pattern formed at a position isolated from the measuring point.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.11/359,374, filed Feb. 23, 2006, which claims priority from JapanesePatent Application No. 2005-049923, filed Feb. 25, 2005, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a pattern measuring method, a patternmeasuring device and a program, and more particularly to a method,device and program for measuring a pattern in an acquired image.

It has been known to measure a pattern on a semiconductor integratedcircuit by using CAD (Computer Aided Design) data. Design data such asCAD data represents intended, ideal geometries of semiconductor devices,so comparison between the CAD data and an actually formed pattern canevaluate a semiconductor manufacturing process. In U.S. Pat. No.6,868,175B1 and U.S. 2002/0015518A1 is disclosed a technology whichdetects an amount of deformation of a pattern with respect to designdata by detecting an edge of a pattern to be inspected and an edge of areference pattern and comparing these detected edges.

As described above, the actually formed pattern exhibits a shapedifferent from that of the design data because of manufacturing processeffects. Many different shapes of patterns are formed on a semiconductorwafer. There is no definite criterion for position alignment between thedesign data and the actual pattern and thus it is not possible tomeasure, according to some reference, a degree to which the patternbeing inspected is deviated from an ideal pattern represented by thedesign data or how much the pattern is deformed.

SUMMARY OF THE INVENTION

To solve the problem described above, a reference position for ameasuring point to be measured by a SEM (scanning electron microscope)and the like is set based on position information of a reference patternon an image acquired from the SEM and based on a positional relation,detected by using design data, between the measuring point and thereference pattern formed at a position isolated from the measuringpoint.

In this construction, since the position of the reference pattern isdetected from the image acquired from the SEM and, with this position asa reference, the reference position for measurement is set using thedesign data, it is possible to evaluate to what extent an actual patternis deviated or deformed from an ideal pattern location or pattern shape,by using the design data and the position information based on a SEMimage.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an outline of a SEM (scanning electron microscope).

FIG. 2 shows circuit design data and a pattern superimposed together.

FIG. 3 is a flow chart from a generation of a dimension measuring recipeto an evaluation based mainly on the circuit design data.

FIG. 4 shows an example of an image acquisition point.

FIG. 5 shows an example of a box-in-box pattern.

FIG. 6 shows where on a chip of a wafer the design data corresponds to.

FIG. 7 shows an example pattern used to determine a distance between thedesign data and a position on the wafer.

FIG. 8 is a diagram showing a positional relation between a firstaddressing point and an image acquisition point.

FIG. 9 is a diagram showing how a second addressing point is searched.

FIG. 10 is a diagram showing a positional relation between the searchedsecond addressing point and the image acquisition point.

FIG. 11 shows a detailed example on the circuit design data of FIG. 10.

FIG. 12 shows an example in which positions of a SEM image and thecircuit design data are aligned at the second addressing point.

FIG. 13 shows an example image acquisition point cut off FIG. 12.

FIG. 14 shows an example in which a measuring point is determined froman image obtained from FIG. 13.

FIG. 15 shows an example in which a distance is measured between animaginary line segment not present on a wafer and a pattern.

FIG. 16A and FIG. 16B show a location of measurement performed in FIG.15 and an intensity of a secondary electron signal at that location,respectively.

FIG. 17 is a flow chart showing a detailed sequence of informationsetting in a SEM recipe.

FIG. 18 is a conceptual diagram of a generated recipe.

DESCRIPTION OF THE EMBODIMENTS

An outline of a SEM (scanning electron microscope) will be explained inthe following. An electrooptical system of FIG. 1 focuses a chargedparticle beam (electron beam) 2 emitted from a charged particle source(electron gun) 1—that releases electrons, or charged particles—by a lens3 onto a specimen 4 and scans the specimen in a desired sequence.Secondary particles (e.g., secondary electrons) 5 produced at thesurface of the specimen 4 as a result of application of the electronbeam are detected by a secondary particle detection system 6, from whichthey are supplied as image data to a control system 7 (controlprocessor) with an image calculation control function. The specimen 4can be moved in all three-dimensional directions by an X-Y-Z stage 8.The control system 7 performs control on the charged particle source(electron gun) 1, lens 3, secondary particle detection system 6, X-Y-Zstage 8 and also on a display 9.

In this example, the electron beam 2 is scanned over the specimen 4two-dimensionally (in X-Y directions) by a scanning coil not shown. Asignal detected by a secondary electron detector in the secondaryparticle detection system 6 is amplified by a signal amplifier in thecontrol system 7 and then transferred to an image memory and shown as aspecimen image on the display 9. The secondary signal detector may beone that detects secondary electrons or reflected electrons or one thatdetects light or X rays.

An address signal corresponding to a memory position on the image memoryis generated in the control system 7 or in a separate computer,converted into an analog signal and supplied to the scanning coil. When,for example, the image memory is 512×512 pixels, an address signal inthe X direction is a repetitive digital signal ranging from 0 to 512 andan address signal in the Y direction is a repetitive digital signalranging from 0 to 512 that is incremented by 1 when the X-directionaddress signal reaches a value of 512. These address signals areconverted into analog signals.

Since the address of the image memory matches an address of a deflectionsignal for scanning the electron beam, the image memory is recorded witha two-dimensional image of a beam deflection area in which the scanningcoil deflects the electron beam. The signal in the image memory can bechronologically read out in sequence by a read address generationcircuit synchronized with a read clock. The signal read out based on theaddress is converted into an analog signal that becomes a luminancemodulation signal of the display 9.

The control system 7 is provided with an input device not shown, bywhich an image retrieve condition (scan speed and total number of imagedpages), a viewing field correction method and an image output andstorage can be specified.

The device of this example has a function to form a line profile basedon detected secondary electrons or reflected electrons. The line profileis formed according to an amount of electrons detected when scanning aprimary electron beam one-dimensionally or two-dimensionally, oraccording to brightness information of the specimen image. The lineprofile thus obtained is used, for example, to measure dimensions of apattern formed on a semiconductor wafer.

While in FIG. 1 the control system 7 has been described to be integralwith the SEM or configured in an equivalent state, it is not limited tothese conditions. For example, processing explained in the following maybe performed by a control processor provided separately from the SEM. Inthat case, it is necessary to provide a transmission medium by which totransfer a detection signal detected by the secondary signal detector tothe control processor or transfer a signal from the control processor tothe lens or deflector of the SEM, and also an input/output terminal toinput or output signals transferred via the transmission medium.

The device of this example also has a function which stores, as a recipein advance, conditions (such as measuring locations and opticalconditions of the SEM) for observing a plurality of points on asemiconductor wafer and performs measurements and observations accordingto a content of the recipe.

It is also possible to register a program for the processing explainedin the following with a storage medium and execute the program by thecontrol processor that supplies necessary signals to the SEM. That is,an example explained below also covers a program or program product thatcan be employed in a charged particle beam device such as a SEM with animage processor.

Further, the control system 7 includes a design data management unit 10which stores design data of a pattern formed on the semiconductor waferand converts it into data required for the control of the SEM. Thedesign data management unit 10 has a function to generate a recipe forcontrolling the SEM based on the design data of the semiconductorpattern entered from an input device not shown. The design datamanagement unit 10 also has a function to rewrite the recipe accordingto a signal transferred from the control system 7. Although, in thisexample, the design data management unit 10 is described to be separatefrom the control system 7, it may be otherwise. For example, the controlsystem 7 and the design data management unit 10 may be integrated witheach other.

This example takes a wafer in the semiconductor product manufacturingprocess as the specimen 4. A resist pattern formed on the wafer in thelithography process was used. For comparison with the resist pattern,semiconductor circuit design data (CAD data) that constitutes a base forthe pattern was used. The semiconductor circuit design data used hererepresents an ideal geometry for the final semiconductor circuitpattern. Although in the following description, the subject to beinspected is taken to be a semiconductor wafer, other subjects may beused as long as there is a correspondence between the design data and asubject to be evaluated. The circuit design data may be of any kind ifsoftware that displays the circuit design data can display its formatand handle it as graphical data.

Embodiment

Conventionally, in a SEM an observer manually specifies measuringpoints. Thus, the locations of the measuring points need to be found onthe wafer but it is very difficult to determine the specified locationson the wafer. After the measuring points are specified, the observermust set conditions necessary to prepare a recipe for the SEM, such asaddressing points and autofocus points, for each measuring point.Therefore, the precision of this manual setting necessarily depends onthe experience of the observer.

Further, since there is a limiting condition that the preparation of thedimension measuring recipe requires a SEM and a wafer, an efficient workhas not been possible. If, for example, a pattern is chosen as anaddressing point, whether that choice is appropriate or not cannot beknown until it is actually matched to the pattern on the wafer. Byrepeating such a trial and error the dimension measuring recipe for theSEM is made. A prolonged time taken by the recipe preparation means areduction in the efficiency. Another factor that should be pointed outfor the bad efficiency of the conventional method is that the SEM mustbe used even during the generation of the dimension measuring recipe,during which time other measurements cannot be made.

As the design rule of semiconductor devices is getting smaller in recentyears, the use of an exposure wavelength shorter than a pattern criticaldimension (CD) necessarily increases an optical proximity effect and anoptical proximity correction technology has a growing significance. Theoptical proximity effect is a phenomenon in which even patterns with thesame reticle CD may have different pattern CDs depending on theenvironment in which the patterns are placed.

The environment referred to here is, for example, a pattern pitch. Underthe condition that a pattern CD on a mask and a pattern CD on a waferare equal in a dense environment, an isolated pattern in a coarseenvironment become narrower in CD. This phenomenon is called the opticalproximity effect and a technology to compensate for this effect is theoptical proximity correction. Increasing the reticle CD of the isolatedpattern can adjust the pattern CD on a wafer to be equal to that of thedense pattern.

Such a simple optical proximity correction can be evaluated by measuringthe pattern CD. In recent years, an evaluation method has been proposedwhich measures a pattern shape in order to make an optical proximitycorrection with higher precision. One such example involves measuring apattern position with respect to a reference.

FIG. 2 represents a case where design data and a pattern on the wafercan be superimposed precisely. In this case, not only can the twodimensions be measured but their positional relation can also bemeasured. Measuring the positions can provide more information than canthe pattern CD. So, the position information can be expected tocontribute to more precise optical proximity correction. Since there isno such reference on the wafer, the pattern positions have beendifficult to measure. But this measurement can be made by overlappingthe circuit design data over the wafer pattern in an image acquired bythe SEM.

This example is briefly explained as follows by using a flow chart ofFIG. 3. First, using the circuit design data, evaluation points to beinspected are determined. For each of the evaluation points, a recipefor making measurements by SEM is generated. According to the recipethus prepared, an image of the evaluation points on the wafer to beinspected is acquired by using the SEM. The acquired image is comparedto the circuit design data for evaluation. While in this example, thecomparison is made with the circuit design data, other referencepatterns may be used as long as they can be compared. For example,comparison may be made between a simulated image obtained from thedesign data and an image acquired by the SEM.

Individual steps in the flow chart of FIG. 3 are explained below one byone. In this example, design data used for evaluation is a simulatedcircuit design pattern with a minimum line width of 90 nm which isgenerated in GDSII format. In a step of determining evaluation points inthis simulated circuit design pattern, a lithography simulator is usedto inspect a wafer following the lithography process.

When inspecting a wafer following other processes, inspection points maybe determined by using an associated simulator. For example, in apattern following an etching process an etching simulator may be used todetermine inspection points. In this example, a lithography simulator ofSigma-C make, Solid-C, was used. This simulator can directly handleGDSII format data of the circuit design pattern and, by using processconditions of the lithography process from the circuit design pattern,it is possible to specify in what pattern the circuit design patternwill be obtained following the lithography process.

A condition used for simulation is: a projection optical systemreduction ratio of ¼, an exposure wavelength of 193 nm, a NumericalAperture (NA) of 0.73, a coherence factor σ of 0.75, a ring shieldfactor ε of 0.67, and a set exposure energy of 28 mJ/cm². This conditionis the same as used for wafers manufactured later. A design rule checkerwas used to select, from a pattern geometry obtained by the simulation,those locations that are considered likely to cause defects. This is atool that automatically detects locations in the pattern formed by thesimulation where the dimensions will become short. In this example, adesign rule checker incorporated in Calibre of Mentor Graphics was used.The design rule checker detects as hazardous points those locationswhere lines are formed to less than 80 nm in width or to more than 100nm. In this example, only the line width are checked for uncertainlocations, it is also possible to extract uncertain locations in otherrespects. The detected uncertain points were output as coordinates onthe circuit design pattern.

That is, for a design rule of 90 nm, lower and upper threshold are setat 80 nm and 100 nm, respectively, and those locations where the linewidth is equal to or in excess of these thresholds are extracted ashazardous points and their coordinates with respect to the design dataare output. In this example, the extracted information is shown on adisplay device or display 9 in the design data management unit 10. Thisarrangement facilitates a decision making as to where on a semiconductorwafer formed with a plurality of different patterns the measuring points(evaluation points) should be set. It is also possible to provide aninput device such as a pointing device in the design data managementunit 10 or control system 7 and to select, from among the uncertainpoints, desired locations as the measuring points. By makingarrangements so that the above selection causes the recipe to beautomatically rewritten, the operator can specify, from among theuncertain points, the locations to be measured based on his or herexperience, realizing both the proper selection of uncertain points andthe more efficient evaluation in terms of time. An arrangement may alsobe made which involves automatically recording the detected uncertainpoints in the recipe as locations to be measured and later deletingunnecessary measuring locations from them.

Next, a recipe is generated for the SEM to inspect the outputcoordinates. To measure the evaluation points by the SEM, it isnecessary to change, according to the environment in which theevaluation points are placed, the set conditions including waferalignment points for correcting not only the coordinates of evaluationpoints but also stage coordinates and chip coordinates, addressingpoints leading to the wafer alignment points used to make positioncorrection step by step, and locations where autofocus and autostigmaare performed.

In this example, areas for the addressing points and autofocus andautostigma points are automatically determined from the design data.

Further, since the design data can be handled as an addressing template,the generation of the dimension measuring recipe has become possiblewithout a wafer on which the dimension measurement is to be performed.The settings of other than the above coordinates (magnification factoron the addressing points and measuring points, acceleration voltage,specimen current, electrooptical system condition and contrastcondition) were specified as the measurement conditions by the observer.This is because these settings cannot be determined from the circuitdesign data but are the information that should be determined by theobserver.

The recipe for the SEM involves first performing the wafer alignment tocorrect the coordinates between chips formed on the wafers. This allowscoordinates on the chips to be used in the SEM. Next, images ofindividual hazardous points are acquired. In this invention theaddressing is performed in steps. The first addressing corrects errorsin the stage precision of the SEM and the next addressing corrects theposition of the patterns to be measured. Since the guaranteed range of astage stopping accuracy in the SEM used in this invention is 4 μm, themagnification at the first addressing point was fixed at 20K, which setsone side of the viewing field (square) to 6.75 μm. It is of coursepossible to perform the addressing with a smaller magnification, i.e.,with a viewing field measuring more than 4 μm on one side. In thisexample, the magnification was set to the one at the first addressingpoint.

As described above, the first addressing point depends on the stagestopping accuracy of the SEM used. The second addressing point isselected within the same viewing field as the final measuring point. Anexample case will be explained as follows in which an image is storedusing 2048×2048 pixels. If individual pixel areas in the SEM image areconsidered equal, the 2048×2048 pixels produce a SEM image in theviewing field four times the side, and 16 times the area, of the viewingfield with 512×512 pixels. The 2048×2048-pixel area includes not onlythe measuring point but also the second addressing point. The finalimage to be acquired is a 512×512-pixel image with one side measuring900 nm. Other magnification factors may be used as long as they producea desired resolving power.

In this example, it is already known in advance that this magnificationfactor produces a desired resolving power, so this magnification factorwas used. Under this condition, the second addressing point is a squarearea which includes a 900-nm square and which measures 4×900 nm, or 3600nm, on each side. Because the number of pixels in each side of theacquired image is quadrupled, the side of the image area is alsoquadrupled. If an image of this square area is acquired and if thepattern acquired by SEM can be matched to the circuit design data atappropriate locations/areas in this square area, then the finallyobtained image lies at a position matching the design data.

FIG. 4 schematically shows hazardous locations output from thelithography simulator and the design rule checker, i.e., a 900-nm squarearea with a point from which an image should be acquired located at thecenter, and a 3600-nm square area including the 900-nm square area. Atthis stage, however, the second addressing point is not yet determined,so the process for specifying the second addressing point will bedescribed later.

For the wafer alignment point that is set at the initial stage, analignment precision correction mark located on the chip is used. Inmanufacturing a semiconductor device, a device layer is laid over animmediately preceding layer to form an intended device. The alignmentprecision correction mark is used to check if the overlapping precisionis within a range of specification and, in the processes following theoverlapping step, the overlapping precision is verified by a dedicateddevice. Representative of this mark is a pattern called a box in box,such as shown in FIG. 5. Two squares are each on separate layers and adeviation between the center coordinates of the two boxes is detected asan alignment accuracy.

Since this pattern is put at a predetermined position on each devicelayer, its coordinates can be specified from the design data. As atemplate image for alignment, design data was used. Other patterns thanthe box-in-box pattern may be used to perform the wafer alignment.

Next, the image acquired by the SEM and the design data must be matchedin position. Generally, there is no particular rules regarding acoordinate origin for the design data. So, unless a correlation valuebetween them is given, there is no association between the coordinatesof the design data and the SEM image. In FIG. 6 a hatched area isassumed to represent the design data and an outer frame a chip of awafer. It is very difficult to handle the circuit design data for thesame area as the wafer chip area because of the data size. In thisexample, the circuit design data was prepared for only a limited area inwhich a point to be measured exists. This arrangement made the data easyto handle.

In order to match the positions of the design data and the SEM image,coordinates of a particular position are determined in each coordinatesystem and their coordinates are made equal. In this example, anH-shaped character pattern of an appropriate size is arranged on thedesign data as shown in FIG. 7, and the coordinates in the two systemsof the same location are used. The SEM pattern, unlike the design data,is curved at the corners, so two line segments are extended and anintersecting point is taken to be the corner.

Under the above conditions, detailed information on the SEM recipe isdetermined. FIG. 17 is a flow chart showing details of an informationsetting sequence. First, a final measuring point and its magnificationfactor are specified (S0001). Although this example first specifies themeasuring point and the magnification factor, others may be specified.

Next, to determine a point for correcting the position of the SEM image,i.e., an addressing point, a candidate area is determined. Morespecifically, the candidate area needs only to be within a range inwhich the viewing field can be changed by moving the electron beam path.If the candidate area is an area extending 15 μm up/down and left/rightfrom the evaluation point as the center, the addressing point isdetermined within this range. In other words, the addressing point needsto be set in an area 30 μm on each side.

Even if the SEM stage stopping accuracy is lowest, when the stage failsto stop within the viewing field, the SEM image cannot be matched andits position not corrected. So, the magnification factor and the viewingfield at the first addressing point depend on the stage stoppingaccuracy of the SEM. The size of the addressing template is determinedwithin a 30 μm square area. In this example, the size of the templateimage of the addressing point is determined by limiting the viewingfield to a square area 6750 nm in one side within the 30 μm square areaand a location suited for addressing is searched as shown in FIG. 8(S0002) (S0003).

In this example, since the side of the viewing field was 6750 nm thatgave a wider area than the stage stopping accuracy. A wider viewing areawill pose no problem at all in terms of system because the templateimage falls in this viewing field if there is an error in the movementof the SEM stage. In this example, a square area 6750 nm in one side wasautomatically determined as a template in the square viewing field 30 μmin one side by applying a normalization correlation method.

While in this example an area suited for addressing was automaticallydetermined by using a method described in “Iwanami Course: MultimediaInformation 5, Information Processing on Image and Space, p. 56”. Theaddressing area was determined by excluding an area of the final imageto be acquired.

In this example, a cross pattern as shown in the figure was determinedas the first addressing point. Next, the autofocus point and theautostigma point were automatically determined from the design data in away similar to that of the first addressing point. The determination ofthe autofocus point and the auto stigmapoint differs from that of thefirst addressing point in that their appropriate areas are determined inthe same viewing field as the final image acquisition magnificationfactor (in this invention, a 900-nm square area).

Next, a decision is made as to whether the area in which the firstaddressing point was set is allowed to overlap the measuring area(S0004). If there is a pattern in the addressing area, the autofocus canbe executed in that area. The operator makes a decision as to whetherthe addressing point should be searched in an area including themeasuring area (S0005) or in an area not including the measuring area(S0006). In the last step, the operator determines the position/area ofthe first addressing point (S0007).

Next, a location for the second addressing point is determined. In a3600-nm square area encompassing the final, required image area of900-nm square, the addressing point needs to be determined by the methoddescribed above in an area excluding the image acquisition point. It isnoted, however, that at the first addressing point one pixel of the SEMimage is about 13-nm square area and that an addressing errornecessarily occurs within the range of this pixel size.

To prevent a possible positional deviation at the second addressingpoint due to this error, a location for the second addressing point issearched in a 3550-nm square area, more than 15 nm inside the 3600-nmsquare area. Since the final acquired image needs only to be included inthe 3550-nm square area, the second addressing point is searched in asquare area measuring 3550 nm×2−900 nm=6200 nm in one side with theimage acquisition point at the center as shown in FIG. 9. In thisexample, the magnification factor (viewing field) at the secondaddressing point was set equal to that of the measuring area (S0008). Ingenerating a dimension measuring recipe, before determining the positionand area of the second addressing point (S0013), the search area for thesecond addressing point is set (S0009) and the operator decides whetherthe second addressing area is allowed to overlap the measuring area(S0010), as in the case of the first addressing point. The operatordecides whether the addressing point should be searched in an areaincluding the measuring area (S0011) or in an area not including themeasuring area (S0012), and then determines the position/area of thesecond addressing point.

If, as a result of the search, the second addressing point is determinedas shown in FIG. 10, a middle point on a line segment connecting thecenters of the image acquisition point and the second addressing pointis taken as the center of the final, acquired image (S0014). Since thefinal image acquisition area is a 3600-nm square area, this areaincludes the second addressing point and is an area of the final imageacquired by the SEM. Further, addressing is performed in this area toextract only a 900-nm square area and overlap the design data and theSEM image, thereby producing a completely aligned, overlapped image.

A recipe for acquiring the SEM image is generated in the mannerdescribed above for each hazardous point output from the processsimulator. The result of automatically determining the second addressingpoint in the 6200-nm square area with the image acquisition point at thecenter is shown in FIG. 11. That is, this area is the mostcharacteristic shape and therefore suited for the addressing point. Fromthe second addressing point and the image acquisition point, the final,acquired image area was determined.

A pattern is formed on a wafer, for example, in a process describedbelow. First, a coat type reflection prevention film is spin-coated onthe wafer to a thickness of about 60 nm. This is further spin-coatedwith a chemically amplified, positive resist film about 200 nm thick.This wafer is masked with a photomask formed from the design data usedin this example and exposed under the same condition as used in theprocess simulator: a projection optical system reduction ratio of ¼, anexposure wavelength of 193 nm, an NA of 0.73, a coherence factor σ of0.75, a ring shield factor ε of 0.67, and set exposure energy of 28mJ/cm². After exposure, the wafer is subjected to a post exposure baking(PEB) at 100° C. for about 90 seconds and then immersed in an alkalinedeveloping liquid of 0.21 N for about 60 seconds for development to forma transferred pattern on the wafer.

An image of hazardous locations on the wafer is acquired by the SEM. Therecipe used for the acquisition is generated from the design data by themethod described above. Since the recipe is based on the design data, ifthe design data is not matched to the position of the image acquired bythe SEM, the addressing point does not function and thus the final,required image cannot be acquired. In this example, a method describedin JP-A-2002-328015 was used as the matching method.

This method makes it possible to use the design data as the template forthe SEM, thus acquiring a SEM image of hazardous points. The result ofmatching the acquired SEM image to the design data in the 3600-nm squarearea including the second addressing point and the hazardous point isschematically shown in FIG. 12. This shows a SEM image in an image areaincluding the second addressing point and the design data correspondingin position to the SEM image and overlapped on it. They are matched inthe viewing field.

In this example, the second addressing point isolated from the hazardouspoints is taken as a reference pattern, and a reference position formeasurement (e.g., line portion of a line pattern at left in FIG. 12) isset based on the information of reference pattern position on the imageand the design data for a portion including the reference pattern andthe hazardous points (points to be measured). As shown in FIG. 12, thesecond addressing point and the position on the SEM image correspondingto the second addressing point are matched, overlapping the SEM imageand the design data. This overlap allows the reference position formeasurement to be set by using the actual position information obtainedfrom the actual SEM image and an ideal position relation between thereference pattern obtained from the design data and the measuredlocation.

Particularly in this example, since the measured location is a linepattern extending vertically, there is a problem that the referenceposition for measurement in the vertical direction in the figure isdifficult to set precisely. However, by taking a pattern extending inboth vertical and horizontal directions in the figure as the secondaddressing point, a position alignment precision in both vertical andhorizontal directions in the figure can be secured, allowing themeasurement reference position in the vertical direction of the figureto be located with high precision. This method of setting themeasurement reference proves very effective, for example, in evaluatingby how much the front end of the line pattern has shrunk with respect tothe design data.

An example dimension measuring method using the measurement referenceposition set by the above method will be explained.

FIG. 13 shows only an image acquisition area extracted from the acquiredSEM image and design data overlapped on the viewing field image.Although in this viewing field, the design data and SEM image seemdeviated in position, their positional relation is correct because theviewing field is cut off a larger area that has been matched with thedesign data.

A SEM image obtained from this figure is evaluated with respect to thedesign data. To make an evaluation in terms of a line width detected bythe design rule checker, the line width of the SEM image in this viewingfield is first measured.

In the case of FIG. 13, it can be evaluated how much the actually formedpattern is deviated from the design data as a reference.

In the case of FIG. 14, a difference between the SEM image and thedesign data is taken to be an evaluation value. Comparison with thedesign data allows for an evaluation in other respects than the linewidth. A distance between a line segment of the design data and an edgesegment of the SEM image can be taken as an evaluation item. In the caseof FIG. 14, it is possible to evaluate how much the pattern has shrunkin the vertical direction or a degree of lateral deviation with respectto the design data as a reference. In the case of a contact holepattern, though not shown, a new contact hole evaluation method can beproposed which makes comparison between the reference positions and theupper and bottom parts of the contact hole. In this case, the referencepositions for both the upper and bottom parts of the contact hole may bemade settable.

Further, data thus obtained may be displayed as a wafer map. An examplemethod for displaying the data as a wafer map is described inJP-A-2001-110862 (U.S. Pat. No. 6,765,204).

These distances are the values that can only be obtained if the SEMimage and the associated design data are overlapped correctly.Additional overlap of the design data corresponding to the next processof the device pattern allows for an evaluation in other respects.

Square design data shown dashed in FIG. 15 represents an ideal positionof a pattern formed in the next process of the semiconductor devicefabrication. Since there is no overlay error for each piece of designdata, a degree of superimposition with the data shown in a dashed linemay be used as an evaluation reference. In this example, a distance wasmeasured between the dashed line design data and the pattern end.Comparison between the SEM image pattern acquired in this manner and thedesign data allowed a new measurement value to be calculated. Parametersrequired for measurement were not determined from the circuit designdata but determined optimally from a SEM image after it was acquired. Inthis example, a linear approximation method was used for the measurementbased on the SEM image. A threshold in this method was set at 50% of amaximum intensity of secondary electron signal.

FIG. 16A shows a measuring point in a SEM image. A secondary electronsignal was detected over an area from the top to the bottom of thefigure. FIG. 16B shows an intensity of a secondary electron signal. Theposition of the right side of the figure is determined by the linearapproximation method with its threshold set at 50%. The position of theleft side of the figure was already determined by the matching betweenthe circuit design data and the SEM image, so the dimension could bemeasured from both of their values. Measurements were able to be made atother locations by the similar method.

FIG. 18 is a conceptual diagram showing a generated SEM measurementrecipe. Such a conceptual diagram is shown on a display during therecipe generation process so that a person making the recipe (operator)can decide whether the image acquisition area is appropriate or not. Inthis example, areas to be set in the recipe generation process are shownon the circuit design data to make it possible to ascertain whetherthere is an overlapping in the region where the electron beam isscanned. Further, displaying these areas in different colors fordiscrimination and indicating their roles allows them to be easilyidentified.

For example, AP1/AF indicates that the area of interest is a firstaddressing area in which the autofocus is to be executed. If theelectron beam scanning is performed for addressing and autofocus beforethe measurement is made, the measured value may differ from that whensuch an electron beam scanning is not performed. Therefore, the aboveindication is effective.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A pattern measuring method for acquiring an image of a pattern on aspecimen and measuring the pattern on the image, the pattern measuringmethod comprising the steps of: acquiring an image of a referencepattern arranged at a position isolated from a measuring point on thespecimen; and setting a reference position for the measurement based ona position of the reference pattern on the image and based on designdata for a portion including the reference pattern and the measuringpoint.